Satellite to satellite microwave communication links commonly found in telecommunication and military space missile tracking application fulfil the need to electronically transfer or "hand off" data and/or other information between different geographic regions on earth being served by respective multiple satellites positioned hundreds of miles apart in geosynchronous orbit respectively covering those disparate regions or positioned in non-synchronous orbits that travel through a respective region.
To implement the communications link, the satellite contains appropriate microwave transmitters and receivers and an antenna that is highly directional in its RF radiation pattern, referred to as a cross-link antenna. The antenna is pointed at a like cross link antenna on another companion satellite in the link with which communication is to occur. Fed with microwave signals from the one satellite's microwave transmitter, the associated antenna, acting as a transmitting antenna, radiates the microwave energy furnished by the transmitter in the direction of the other satellite's antenna. The latter antenna, functioning as a receiving antenna, receives and couples the microwave signals to an associated microwave receiver on the companion satellite.
Concentrating the transmitted radiation in a specific direction to a known location offers the most efficient use of available transmitter power. Like concentration of received energy offers the most efficient reception by minimizing reception of out-of-direction unwanted signals originating elsewhere that might interfere with the desired communication, and, therefore, also minimizes transmitter power requirements.
One type of directional antenna achieving wide acceptance in the foregoing application and often found to be the antenna of choice is known as a dual offset Cassegrain antenna. That antenna comprises a parabolic reflector as a main reflector, a hyperbolic reflector as the subreflector and a microwave feed located at three spaced positions.
In a typical dual offset type Cassegrain antenna, the feed is positioned on the paraboloids focal axis spaced a distance from the hyperbolic reflector equal to the focal distance of the hyperbola's foci and the axis of the feed is tilted up. Both subreflector and feed are separately off-set in different amounts from the antenna's aperture that is defined by the main reflector.
In past circumstances where the size of the antenna is found to exceed the space available to store same on board the satellite, a typical approach was to include a hinged section in the antenna assembly. The hinge allowed the antenna to be folded down to fit within a smaller volume. When the satellite was positioned in space, the antenna was unfolded by means of a motor mechanism or robot arm to achieve its full size. Although such a foldable antenna satisfied existing requirements, not only does the incorporation of a hinge and related motors undesirably increase the weight of the assembly, the antenna elements may not precisely align to the desired relationship, resulting in poorer performance; and in some circumstances the mechanisms that were to unfold the antenna failed resulting in the failure of the entire mission.
Accordingly, an object of the invention is to provide a dual offset Cassegrain type antenna that is smaller in size than prior antennas of that type for the same transmitting frequency while delivering no less performance and, preferably, that delivers improved performance.
Another object of the invention is to provide an antenna for V-band operation that avoids the use of hinges and other implements characteristic of a foldable construction.
Still another object of the invention is to provide a low cost and light weight V-band cross link antenna system having a gain greater than 45 dB, a high efficiency, greater than 73%, and low side lobes.